1. Field of the Invention
The present general inventive concept relates to a horizontal deflection circuit, and more particularly, to a horizontal deflection circuit capable of controlling a base current of a horizontal output transistor according to a frequency mode, thereby decreasing damage to the horizontal output transistor.
2. Description of the Related Art
Generally, a cathode ray tube (CRT) used in an image displaying apparatus utilizes a principle that respective electron beams having different amounts beat monochrome or fluorescent materials of Red, Green and Blue (RGB) coated on a surface of the CRT according to different intensities of image signals, thereby producing lights different in brightness or color.
A CRT image displaying apparatus receives an image signal and a synchronous signal input from a video card of a computer system and displays information on a screen. The CRT image displaying apparatus comprises a video system to process image signals, a deflection system for vertical and horizontal deflection, and a power system.
FIG. 1 illustrates a conventional horizontal deflection circuit of a CRT image displaying apparatus.
Referring to FIG. 1, the conventional horizontal deflection circuit comprises a horizontal driving signal generating unit 100 generating a horizontal driving signal of a square wave based on a horizontal synchronous signal input from a microcomputer (not shown), a horizontal driving unit 200 amplifying the horizontal driving signal generated from the horizontal driving signal generating unit 100 to a predetermined level, and a horizontal output unit 300 generating a sawtooth wave current based on the horizontal driving signal amplified in the horizontal driving unit 200 and supplying the sawtooth wave current to a horizontal deflection coil (HD.Y′).
The horizontal driving signal generating unit 100 signal-modulates the horizontal synchronous signal output from the microcomputer and then supplies the signal-modulated horizontal synchronous signal to the horizontal driving unit 200 as the horizontal driving signal.
The horizontal driving unit 200 comprises a buffer circuit 202 buffering the horizontal driving signal generated in the horizontal driving signal generating unit 100, a horizontal driving transistor Q1 being turned on or off according to the horizontal driving signal ouput from the buffer circuit 202, and a horizontal driving transformer HDT′ accumulating energy therein according to the turning on or off of the horizontal driving transistor Q1.
The horizontal driving signal is supplied to a base terminal of the horizontal driving transistor Q1 through the buffer circuit 202. Then, the horizontal driving transistor Q1 is turned on in the on section of the horizontal driving signal, and the horizontal driving transformer HDT′ thereby accumulates energy therein. The horizontal driving transistor Q1 is turned off in the off section of the horizontal driving signal, thereby transmitting the energy accumulated in the horizontal driving transformer HDT′ to the horizontal output unit 300. Here, resistors R1 and R2 are used to control an amount of current.
The horizontal output unit 300 comprises a horizontal output transistor Q2, a damper diode D1, a return capacitor C1 and the horizontal deflection coil HD.Y′. The horizontal output transistor Q2 is turned on or off according to an intensity of the current output from the horizontal driving unit 200.
The horizontal output transistor Q2 is turned off while the horizontal driving transformer HDT′ accumulates energy therein, and is turned on while the energy accumulated in the horizontal driving transformer HDT′ is being transmitted to the horizontal output unit 300.
When the horizontal output transistor Q2 is turned on, B+ power current is supplied to the horizontal deflection coil HD.Y′.
However, when the horizontal output transistor Q2 is abruptly turned off, the current accumulated in the horizontal deflection coil HD.Y′ is charged through the return capacitor C1. When the return capacitor C1 is completely charged, the voltage charged in the return capacitor C1 is again discharged into the horizontal deflection coil HD.Y′ and subsequently the current is again accumulated in the horizontal deflection coil HD.Y′.
When energy is accumulated in the horizontal deflection coil HD.Y′ to the degree that the voltage of the horizontal deflection coil HD.Y′ applies a bias in a forward direction to the damper diode D1, the damper diode D1 is electrically conducted and the current flowing on the horizontal deflection coil HD.Y′ is dropped to zero (0).
At the point of time when the current flowing on the horizontal deflection coil HD.Y′ reaches zero (0), the horizontal output transistor Q2 is again turned on by the horizontal driving unit 200 and the steps described above are repeated.
Through these steps, as the sawtooth wave current is supplied to the horizontal deflection coil HD.Y′, horizontal deflection is attained and horizontal scanning is made.
However, where two or more frequency modes including a DTV mode (28.1 kHz in a case of an European type) or a common RF mode (31.5 kHz) are simultaneously used in an image displaying apparatus having the conventional horizontal deflection coil, the current induced to a second side of the horizontal driving transformer HDT′ increases as a higher horizontal frequency mode is input, thereby increasing the base current of the horizontal output transistor Q2. This causes heat to be generated from the horizontal output transistor Q2. In a case of excessive heat, the horizontal output transistor Q2 can be damaged.